Engine oils degrade during normal use from a variety of mechanisms. One mode involves oil oxidation and nitration from exposure to oxygen and combustion gases at elevated temperatures in the combustion chamber and the oil sump. Another mode involves contamination of the oil by combustion by-products. The rate of oil degradation may depend on engine operating conditions, ambient temperatures, engine design, and oil quality. If the oil is exposed to engine operating conditions beyond a certain point, a rapid increase in the oil degradation rate may occur, and sludge-forming products may begin to deposit on engine surfaces. The goal of the automotive manufacturer is to maximize the oil change interval for environmental and customer cost/convenience benefits, while ensuring that the oil does not degrade to the point of compromising engine performance or longevity.
One common result of engine oil degradation may be a gradual increase of oil viscosity. The direct measurement of viscosity change may provide a first order estimate of remaining oil life and the onset of rapid degradation. However, viscosity may change for reasons other than oil degradation, such as the top-up addition of oil with a different viscosity grade, or fuel contamination in the oil due to repeated operation at colder temperatures. The on-board measurement of viscosity hysteresis may have the potential to provide a more robust means of determining remaining oil life and of detecting the onset of rapid oil degradation.
Viscosity hysteresis may be experienced by engine oils during thermal cycling, i.e. during heating from a given ambient temperature to an operating temperature, typically in the range of 80 degrees Celsius to 150 degrees Celsius, followed by cooling down to the ambient temperature after engine shut-off. Viscosity hysteresis in engine oil may be a function of the oil's composition and its degree of oxidation during use in an engine. The absolute value of the hysteresis may depend on the degree of oil degradation, wherein the higher absolute value may be an indication of an increase in oil degradation. A change in sign of the hysteresis from negative to positive may also signal the moment when the oil degradation reaches the point when formation of varnish and sludge precursors begins.